In effecting a processing such as welding, cutting, surface treatment or the like by a laser processing device, the laser beam is emitted from a laser oscillator and the path of beam is changed by a reflective mirror, while focusing the beam by a lens on the work to be processed. The power of the laser beam emitted from the oscillator affects the workability of the work. It is, therefore, necessary to measure the power of the laser beam. One of the known methods of measuring the power is to use a detachable full-reflecting mirror, a "shutter", in the path of the laser beam and to measure the laser beam reflected by this mirror using a power meter. This method, however, cannot be used without interrupting the processing, because the beam path is intercepted during the measurement of the power.
Japanese Patent Laid-open No. 99792/1978 shows, particularly at FIG. 1 attached thereto, a method of continuously monitoring the power of the laser beam. This method makes use of a half-transmissive mirror adapted to reflect a part of the laser beam and the reflected beam is monitored. This method is also unsatisfactory in that the power of the laser beam is lost undesirably by the half-transmissive mirror disposed in the path of laser beam.
Furthermore, this type of laser beam device involves a problem that the path of the laser beam to be directed to the work is undersirably deviated due to slight inclination or offset of the mirrors incorporated in the optical system, attributable to a temperature change. This problem is serious particularly in the laser device for production purposes, because the laser source in such device is usually located at a distance from the work to inconveniently amplify the offset of the beam path. Namely, even a small deviation at the laser source causes an unacceptably large offset at the position of the work. This offset of the beam path causes not only the position of application of the beam onto the work, but also a deviation of a part of the laser beam from the effective plane of the optical system resulting in a scattering of the laser beam, i.e. a lowered efficiency of use of the laser power, to deteriorate the processing performance of the laser power.